388 CHEMICAL ENGINEERING VARIOUS TYPES OF HOT AIR AND GAS DRYERS Drying chambers are generally used for small quantities, and in cases where most of the work is done over night, without supervision. Fuel and steam consumption is therefore rather high. Figure 21 shows a standard chamber dryer with upward direction of the air. Figure 22, the same type with downward movement, which is some times desirable to reduce the speed of the air 1 1 1 -V 1 I J l~°" — -4 ? FIGS. 21, 22, 23.—Chamber dryers with upward air current. FIG. 24.—Tunnel dryer. FIG. 25.—Tunnel dryer re-using air. current, and avoid pockets and eddies. Figure 23 shows a dryer where the air is always reheated after each passage through one section; it is really a combination of a number of single dryers placed in series. Figure 24 describes a standard tunnel dryer, and Fig. 25 gives the same type with the provision that the air can be used over again. Figure 26 shows a special type where the air is cooled after every passage to remove the moisture by condensation. Such a dryer will require somewhat less steam, but the gain in efficiency is mostly lost by the extra expense for cooling water.